In electrochemical cells such as electrolytic cells or fuel cells, it is important to provide a separator between the anode and cathode compartments. Chemically stable ion exchange membranes made from perfluorosulfonic acid polymer, as described in U.S. Pat. Nos. 3,282,875; 3,718,627; 4,358,545; and 4,329,434, or from perfluorocarboxylic acid polymer, as described in U.S. Pat. Nos. 4,131,740 and 4,734,170, have found broad use as separators, particularly for the electrolysis of brine.
For reasons of quality, efficiency, cost effectiveness and often safety it is important that the separator in an electrolytic cell be tear, abrasion, puncture and scratch resistant, yet not so thick or reinforced that its resistance to ionic conduction is excessively high, in which case the corresponding power requirements of an electrolytic cell are excessively high.
Fluorinated ion exchange membranes are also well known in the field of fuel cells. Such ion exchange membranes have good chemical and thermal resistance and have been used in fuel cells, such as methanol-air fuel cells and hydrogen-oxygen fuel cells. In a fuel cell system, a problem arises which is similar to the problem in an electrolytic cell: reduced electrical energy efficiency arises due to ohmic loss by electrical resistance of the membrane in which case the corresponding power output of the fuel cell is excessively low.
In order to reduce the membrane resistance, it is desirable to decrease the thickness of the membrane and increase the water content. However, a decrease in thickness and an increase of the water content may reduce the electrical resistance, but brings about an abrupt deterioration of the membrane strength.
The mechanical strength of membranes used in an electrolytic cell is important. It is also important in a fuel cell, especially a gaseous fuel cell. Many fuel cells operate at high differential pressure which increases the likelihood of damage to the fragile membrane. Such differential pressure may be subject to fluctuation which also increases the liklihood of damage to the membrane. In addition, fuel cells, especially fuel cells used in motor vehicles or similar applications, are frequently shut down and restarted; such cycling causes the membrane to dehydrate and rehydrate which causes stress and further increases the likelihood of damage to the membrane.
Prior art fluorinated membranes can be in the form of a reinforced or unreinforced film or laminar structure. Use of reinforcement within a membrane, while making it stronger, is not totally beneficial. As noted above, one deleterious effect is that use of reinforcement such as fabric results in a thicker membrane, which in turn leads to higher electrical resistance. A second deleterious effect, which also leads to higher resistance, is caused by a "shadowing" effect of the reinforcing members. The shortest path for an ion through a membrane is a straight perpendicular path from one surface to the other surface. Reinforcement members are usually fabricated of substance which is not ion-permeable. Those parts of a membrane where an ion cannot travel perpendicularly straight through the membrane, and from which the ion must take a circuitous path around a reinforcing member, are termed "shadowed areas." Introduction of shadowed areas into a membrane by use of reinforcement leads to a reduction in the portion of the membrane which actively transports ions, and thus increases the resistance of the membrane. A third deleterious effect of the use of reinforcement within a membrane is poor water management. Particularly, water is being generated in the cathode side of the fuel cell and diffuses through the membrane from the cathode side to the anode side. With a thick membrane having reinforcement, the water diffusion is slow and one side of the membrane dries out, resulting in increased membrane resistance.
Reinforcement mechanisms have been devised in which a fabric, usually made from polytetrafluoroethylene (PTFE) fibers or expanded polytetrafluoroethylene (EPTFE) fibers, has been partially or wholly encapsulated by or embedded in the perfluoro ion exchange polymer. Commercial products reflect this approach. Such reinforced membranes are described in U.S. Pat. No. 4,604, 170; and Japanese Patent Applications No. 62-280231 and 62-280230. However, it requires about a 5-10 mil thickness of ion exchange membrane to effectively bond to and encapsulate the fabric. Thinner membranes are unsatisfactory because they may not completely cover the fabric on both sides and the integrity of the membrane is impaired. The electrical resistance in aqueous media of this reinforced 5-10 mil structure is considerably higher than that of an unreinforced thinner membrane because of the increased thickness and the reduced effective cross section available for ion transport because of the encapsulated fabric. With such a membrane, the resistance is high and such a membrane is not necessarily satisfactory for use in electrolytic cell or fuel cells.
More significantly, PTFE, EPTFE or similar reinforcements are too expensive and are difficult to process. For example, ion exchange membranes reinforced with PTFE or EPTFE may be formed by laminating at an elevated temperature the PTFE/EPTFE to a precursor perfluorinated ion exchange film.
The precursor perfluorinated ion exchange polymer is extruded at a temperature less than 300.degree. C. to form a film. As noted above, this film can be perfluorinated sulfonyl fluoride polymer, perfluorinated carboxylester polymer, or a multilayered structure of such sulfonyl fluoride polymers, carboxylester polymers or both, where the different polymers form distinct layers in the coextruded film.
Lamination of the film (single or multilayer) to the PTFE/EPTFE, takes place with surface temperatures of about 270.degree. C.-280.degree. C. and under a pressure differential of not more than about 760 mm mercury. This lamination step is difficult to control and may result in poor reproducibility and poor uniformity. In addition, the process used to make membranes reinforced with materials such as PTFE or EPTFE disadvantageously requires at least one costly heat lamination step. In addition, the fluorinated membrane may be damaged during such high temperature processing.
Reinforced ion exchange membranes are also useful in chemical separations and facilitated transport mechanisms. In the separation of fluids, membranes through which fluids have different permeation rates have been useful in separating mixtures of those fluids. Such membranes have been wound with macroporous separating meshes which permit free flow of fluids to and from the membrane's surfaces and modules have been constructed. Thin perfluoroionomer films have a very high permeability to water and some other polar molecules, but effective permeation separation modules cannot be built from these thin, fragile perfluoroionomer films.
Facilitated transport is a related separation technique wherein a continuous membrane is plasticized or swollen with a liquid. The dissolved liquid complexes with one of the fluids, such as gases, to be separated and selectively facilitates its transport across the membrane. Again, thin perfluoroionomer films offer some unique opportunities for facilitated transport, as, for example, in the separation of amino acids in aqueous media, but the thin perfluoroionomer does not have sufficient dimensional stability or mechanical strength to undergo module construction or withstand operating pressure differentials. U.S. Pat. No. 4,194,041 provides for a waterproof article which is composited with a hydrophobic EPTFE layer and permits the passage of water vapor. The ability to transport aqueous liquids is important not only in electrolytic processes but also in permeation separation and facilitated transport operations.
The disadvantages of membranes reinforced with PTFE, EPTFE or the like may be overcome by using a membrane reinforced with other porous substrates, such as porous substrates made from hydrocarbons, such as polyolefin, especially polyethylenes, polyesters or polycarbonates, preferably made from linear high density polyethylene ("LHDPE"). Such substrates or fabrics provide a relatively inexpensive membrane with good mechanical strength capable of operation at low electrical resistance with good water management. However, such substrates usually cannot be processed at the high temperatures used to process PTFE/EPTFE reinforcement and the accompanying precursor polymer film. Such substrates may be thermally unstable and may, for example, degrade, decompose or melt at temperatures of 260.degree. C. LHPDE, for example, will melt and lose porosity, and possibly degrade, at temperatures of about 130.degree. C.
Therefore, a method is needed to produce a substantially non-porous, composite ion exchange membrane which is reinforced with porous LHDPE or the like and may advantageously be processed at room temperature or low temperatures to prevent the substrate from degrading or melting.
The present invention provides a simple, inexpensive, reproducible process of making a reinforced substantially non-porous membrane with satisfactory mechanical strength and very low resistance to ionic conduction which approaches that of very thin, unreinforced perfluoro ion exchange polymer membranes. The process eliminates the need for a costly heat lamination step and provides an alternate method for making membranes when high temperature lamination will melt, degrade or decompose on the components. The process of the present invention provides composite membranes which overcome the mechanical strength limitations of thin perfluoroionomer films without significantly reducing the high permeation and transport rates possible with these thin perfluoroionomer films. Such reinforced ion exchange membranes are particularly useful in fuel cells.